1. Field of the Invention
The present invention relates to an air flow quantity measuring apparatus for an internal combustion engine.
2. Description of Related Art
For example, with reference to FIGS. 8A and 8B, Japanese Unexamined Patent Publication JP2009-186417A teaches an air flow quantity measuring apparatus, which includes an air flow meter 101 and an engine control unit (ECU) 102. The air flow meter 101 includes a control circuit 101a and a frequency converter circuit 101b. The control circuit 101a outputs an electrical signal (an electrical signal of a resistance value or voltage value), which corresponds to a sensed value of an air flow quantity of intake air to be supplied to a combustion chamber of an internal combustion engine (hereinafter also simply referred to as an engine) of a vehicle (e.g., an automobile). The frequency converter circuit 101b converts the electrical signal, which is received from the control circuit 101a, into a pulse frequency signal and outputs the pulse frequency signal to the ECU 102. The ECU 102 includes a microcomputer 102a (implementing functions of an edge sensing means for sequentially sensing generation of a falling edge of the pulse frequency signal, a time data storage means for sequentially storing a time data record indicating a time point of generating the falling edge, an edge interval sensing means for sensing an interval time period between the time points, an average period sensing means for sensing an average period of the pulse frequency signal and an air flow quantity sensing means for computing the air flow quantity based on the average period of the pulse frequency signal) to compute the air flow quantity based on the pulse frequency signal, which is outputted from the air flow meter 101.
In the air flow quantity measuring apparatus described above, the period (duration of one cycle) of the pulse frequency signal, which is outputted from the air flow meter 101, is converted into the air flow quantity by the ECU 102 (more specifically the microcomputer), and this air flow quantity is used as a measurement value of the air flow meter 101. In this way, a conversion time period, which is required to convert the pulse frequency signal into the air flow quantity, can be shortened.
However, in the vehicle (e.g., the automobile), the electromagnetic interference (EMI) occurs on the ECU.
In the case where the ECU 102 of Japanese Unexamined Patent Publication JP2009-186417A is under the influence of the EMI, a noise (hereinafter referred to as an EMI noise) overlaps with the frequency signal, which is outputted from the air flow meter 101.
In such a case, the ECU 102 may erroneously converts the pulse frequency signal into the air flow quantity or may erroneously compute the air flow quantity from the pulse frequency signal. That is, when the EMI noise overlaps with the frequency signal, a sensing error of the air flow quantity becomes large, so that an accuracy and reliability of the measurement value of the air flow meter 101 may possibly be deteriorated.